The present invention concerns a procedure and a means for measuring the flowing friction of liquid substances with high internal friction and viscosity, such as molten plastic mix for instance.
Plastic extruders, or extrusion presses, are used to manufacture a huge variety of plastic components, such as tubes and pipes, sewer pipes of PVC, cable insulations of polyethylene, etc. In the extrusion process the granular raw material is placed under high pressure (200 bar) and high temperature (about 190.degree.-230.degree. C.) with a screw press and heating resistances. The molten plastic mix is then pressed through the nozzle or tool, being at the same time formed to become for instance a hollow tube, e.g. a sewer pipe. As a result of the high pressures and temperatures, quite remarkable amounts of energy have to be expended in extrusion processes. In the lack of reliable measuring pick-ups the extrusion process could not be fully optimized up to now. Optimization requires a reliable feedback as basis for control, but a continuously operating control loop could not be successfully implemented heretofore. This is because it has been found that measurement of temperature and pressure alone is not sufficient to base the control on. The state of the melt mass can be estimated to a certain degree by measuring the velocity of ultrasound through the mass, but this too does not disclose the height of friction in the extruder.
In order to optimize the energy consumption, one ought to know the amount of friction energy in the extrusion process. If the raw material is comparatively coarse hard PVC, the contribution of friction is assuredly high. Furthermore, the friction may also significantly depend on the degree of smoothness of the extruder walls. Although on the other hand conjectures are presented to the effect that the wall roughness has no effect on friction for the reason that close to the wall the velocity of the melt mass is low at all events. The flow profile of the melt mass is very poorly understood for the reason that practical measurements are very awkward.